1. Field of the Invention
The present invention relates to a color image forming apparatus, which forms a color image based on an image signal and adjusts color of the color image.
2. Description of the Related Art
In recent years, in electrophotographic or inkjet color image forming apparatuses, such as color printers and color copying machines, an output image of higher image quality has been demanded. In general, stability in density and gradation of images has a significant impact on perceptions of image quality as judged by humans. To assure constant gradation-density characteristics irrespective of variations in the apparatus, conventional density control (“monochromatic control”) includes forming patches of respective color toners on an intermediate transfer member and detecting the density of each patch with an unfixed toner density detection sensor (density sensor).
In this case, stable images can be obtained by performing density control by reflecting detected density information, as feedback, to process conditions (e.g., exposure amount conditions and development bias conditions) serving as image forming conditions.
However, “monochromatic control” (i.e., density control using an unfixed toner density detection sensor) includes formation and detection of patches on an intermediate transfer member or a photosensitive drum, without considering any changes in image color balance when the image is later transferred and fixed on a transfer material. The color balance also changes depending on the efficiency in transferring toner images onto a transfer material or conditions in a fixing process applying heat and pressure to toner images. The monochromatic control cannot correct such changes in color balance.
Hence, another conventional method includes providing a density or chromaticity sensor (hereinafter, referred to as “color sensor”) capable of detecting the density of monochromatic toner images or the chromaticity of full-color images transferred and fixed on a transfer material, forming color toner patches on the transfer material, and detecting the density or chromaticity of the patches with the color sensor to control the density or chromaticity. The method includes reflecting the detected density or chromaticity information, as feedback, to process conditions (exposure amount, process conditions, lookup tables, etc.) and performing density or chromaticity control on a final output image formed on the transfer material.
In general, a color sensor includes a light source (light-emitting element) capable of emitting red (R) light, green (G) light, and blue (B) light, and is configured to identify C, M, Y, and K colors or detect density or chromaticity of each color. For example, the light-emitting element includes a light source that emits white (W) light, on which three types of filters of red (R), green (G), and blue (B), which are different in spectral transmittance, are formed. The color sensor can identify C, M, Y, and K colors and detect the density of each color based on three different outputs (e.g., RGB outputs) obtained by the light-emitting element. The RGB outputs are subjected to linear conversion or lookup table (LUT) conversion to obtain chromaticity expressed in a general color system of L*a*b* or XYZ. In addition, the word “chromaticity” in the present specification is an all-inclusive term for quantitatively representing color. Instead of the word of “chromaticity”, it can be also described as “color information” or “color value”. Further, “chromaticity” can be described merely as “color”.
The color sensor, when used to detect the absolute density (or absolute chromaticity) of a patch, requires a white reference board usable to correct sensor output or a comparable member whose absolute density (or chromaticity) value is known. The first reason to use such a reference member is necessity of correcting dispersion in spectral characteristics of sensor elements (e.g., the light-emitting element and the light-sensitive element). The second reason is variations in sensor output due to aging of the sensor elements (e.g., the light-emitting element and the light-sensitive element) or ambient temperature change. The third reason is reduction in sensor output due to paper dust, toner, and ink deposited on a sensor surface when many transfer materials pass by the sensor in ordinary printing operations. However, the white reference board (i.e., the reference member used for sensor output correction) is expensive and cannot be used anymore if a board surface is soiled by paper dust, toner, or ink.
Hence, another conventional method includes detecting a process gray patch formed by cyan, magenta, and yellow colors and a monochromatic gray patch formed by black color with a color sensor and comparing chromaticity (L*a*b*, L*c*h*, or XYZ) of detected patches. The color of a gray patch formed by black toner usable for electrophotographic printers or black ink usable for inkjet printers is achromatic. Hence, the method includes forming a gray patch of black color as a reference patch every time the density or chromaticity control is performed, and performing correction to match the process gray with the color of the reference patch to obtain a process gray of achromatic color, without using any reference required for sensor output correction (hereinafter, referred to as “gray axis correction control”).
The gray axis correction control does not require preparation of high resolution filters as well as a sensor output correction reference. Therefore, the gray axis correction control can be easily applied to a printer. Furthermore, if the gray balance of cyan, magenta, and yellow is inappropriate, problems such as “gray color (i.e., color most sensitive to human eyes) looks like other color” and “misregistration in tint of a chromatic color” may occur. In this respect, adjusting the process gray to achromatic color and obtaining proper gray balance is effective to solve the above-described problems.
In view of the above-described problems, as discussed in Japanese Patent Application Laid-Open No. 2003-107830, a conventional method obtains an optimum ratio of cyan, magenta, and yellow that can accurately realize a process gray of achromatic color based on a chromaticity value detected by a color sensor. The method includes forming patches by adjusting densities of cyan, magenta, and yellow colors constituting a process gray to attain chromaticity similar to that of a target black.
Furthermore, the method includes calculating an optimum ratio of gradation levels of cyan, magenta, and yellow colors constituting a process gray closest to the chromaticity of the target black, based on chromaticity information detected by the color sensor, using linear interpolation or multiple regression calculation. However, an experimental result revealed that the conventional method could not obtain proper gray balance due to differences in the type of transfer material (plain paper, glossy paper, etc.) and the type of print mode.